There has been a problem that it is difficult to perform stable reception in a coherent receiver without digital signal processing due to an offset of the frequency and the phase of the local oscillation light or a polarization fluctuation.
With regard to this, for example, PTL 1 discloses a technique in which, in a coherent receiver, the amount of the frequency shift between a carrier wave frequency of an input signal and the local oscillation light is estimated and the oscillating frequency of the local oscillator (LO) is shifted in a direction opposite to the estimated value.
On the other hand, because of the development of electronic device technologies, it has become possible to use a high-speed A/D (analog to digital) converter for signal processing in a high-speed communication device. As a result, a digital signal has been transmitted and received in optical communication or radio communication. By performing digital signal processing on a signal converted into a digital signal, it is possible to compensate the above-mentioned offset of the frequency and the phase of the LO light or polarization fluctuation of optical signals. In addition to the above-mentioned compensation of the offset of the frequency and the phase or the compensation of the polarization fluctuation, it is also possible to perform a more advanced waveform equalization technique by digital signal processing.
For example, NPL 1 discloses a technique to compensate for various waveform distortions, which are included in received signals, such as chromatic dispersion and polarization mode dispersion based on the analysis results obtained by analyzing transfer characteristics from filter coefficients of a time domain equalization filter. PTL 2 discloses a technique to compensate for a waveform distortion having semi-fixed characteristics varying due to temperature fluctuation in the preceding stage, and compensate for a fast-varying waveform distortion occurring in a transmission line in the subsequent stage. Other related arts are also disclosed in PTL 3, PTL 4, or the like.